The influence of ultrasonic welding on the crystallinity degree at the welding interface of carbon fibre reinforced polyphenylene sulphide (CF/PPS) joints was investigated. Two sets of welding force and vibration amplitude were used, (1000 N, 86.2 μm) and (300 N, 52.8 μm), representing short and long welding times, respectively. The evolution of temperature with time at the centre of the joint overlap was recorded using thermocouples while the crystallinity degree of PPS was measured using differential scanning calorimetry (DSC). The cooling rate dependency of crystallinity was determined through fast scanning calorimetry (FSC) measurements. It was found that high force and high amplitude resulted in faster cooling rates and predominantly amorphous PPS, while low force and low amplitude resulted in slower cooling rates and yielded PPS of moderate crystallinity. It is suggested that the capability of PPS to crystallize despite the very fast cooling rates could be attributed to strain-induced crystallization during the welding process.